A Group III-nitride semiconductor is a compound semiconductor containing a Group-III element and nitrogen. Group III-nitride semiconductors are promising materials for use in semiconductor devices including semiconductor light-emitting devices, such as laser diodes (LDs) and light-emitting diodes (LEDs), and metal-semiconductor field-effect transistors (MESFETs). In particular, laser diodes that use the Group III-nitride semiconductor gallium nitride (GaN) as their semiconductor material are known to be capable of remarkably improving the read and write capabilities of data storage devices. This is because GaN has a wide band-gap. Consequently, a laser diode based on GaN generates light at wavelengths in the blue-ultra violet range. Wavelengths in this range are short compared with those of light generated by conventional laser diodes that emit red or green light. For example, GaN-based blue-emitting laser diodes are being considered for use in high-capacity Digital Versatile Disks (DVDs) that will be introduced around the year 2000. Such DVDs are planned to have a capacity of about 15 gigabytes.
The lifetimes of current GaN-based laser diodes are short at the light intensities required for practical applications including the above-mentioned high-capacity DVDs. Currently-obtainable lifetimes will have to be substantially increased if GaN-based laser diodes are to have practical value. The major cause of the short lifetime of current GaN-based laser diodes is heat generated as a result of the high contact resistance of the electrical contacts to the GaN of the device. The high contact resistance is a consequence of the large band-gap of GaN. The band gap of GaN is 3.4 eV, which is higher than that of most other semiconductors used for making electronic devices.
FIG. 1 shows an example of a GaN-based edge-emitting laser. The laser 1 is formed by depositing the GaN buffer layer 3 on the sapphire substrate 2 and then successively depositing the n-type GaN contact layer 4, the n-type AlGaN cladding layer 6, the n-type GaN layer 7, the InGaN multi-quantum well layer 8, the p-type GaN layer 9, the p-type AlGaN cladding layer 10, and the p-type GaN contact layer 11. The n-electrode 5 together with the n-type GaN contact layer 4 collectively form the n-contact 13, and the p-electrode 12 together with the p-type GaN contact layer 11 collectively form the p-contact 14. The exposed surfaces of the device are passivated by the SiO.sub.2 layer 15.
The contact resistance of the n-contact 13 composed of the n-type GaN contact layer 4 and the n-electrode 5 is fairly low. However, the contact resistance of the p-contact 14 between the p-type GaN contact layer 11 and the p-contact electrode 12 is much higher than that of the n-contact. For example, the sheet contact resistivity of the p-contact of a typical device having Au/Ni electrodes is about 2.times.10.sup.-1 .OMEGA.cm.sup.2. If the electrode contact area is 300 .mu.m.times.300 .mu.m, the contact resistance of the p-contact is greater than 200 .OMEGA.. Consequently, a drive current of 100 mA will generate more than 2 W in the p-contact.
NIKKEI ELECTRONICS No. 671, p.9, (Sep. 23, 1996), (Nikkei McGraw-Hill Co.) describes a laser diode that has a light-emitting layer having an InGaN multi-quantum well structure composed of 25 quantum wells. A drive voltage of 20 V and a drive current of 5A, corresponding to a power of 100W, is required to cause this device to emit violet light at a wavelength of 417 nm. Since the laser diode is incapable of continuously dissipating this amount of power, it cannot be operated continuously. Instead, it must be operated in a pulse mode with a duty cycle of about 0.001. This corresponds to an average power of 100 mW.
In other applications, contact resistance of the order just described increases series resistance, increases power consumption, increases the operating temperature of the device, degrades the operating efficiency of the device, and shortens the lifetime of the device. What is needed is a reduction in the contact resistance of the metal-to-semiconductor contact to a level that can be ignored compared to the bulk series resistance of the semiconductor device.
The contact resistance R.sub.c of a metal-to-semiconductor contact can be used as a performance index for the contact. Since the contact resistance R.sub.c depends exponentially on the effective barrier height .phi..sub.B between the metal electrode and the semiconductor material, a way of reducing the effective barrier height .phi..sub.B is required. Moreover, in a semiconductor having a high carrier concentration N, where the tunnelling current is dominant, since the contact resistance R.sub.c depends exponentially on the term .phi..sub.B N.sup.-1/2, the contact resistance can be reduced by increasing the dopant concentration N.
The above-mentioned NIKKI ELECTRONICS article describes one conventional technology in which the p-GaN carrier concentration is increased to more more than 10.sup.19 cm.sup.-3. However, this technique lead to a marked decrease in the acceptor activation rate and produces an extreme degradation of crystallinity. Consequently, this technique does not produce favorable results.
In Electrical Characteristics and Interface Structure of an Ni/Au Contact on P-type GaN, PROC. OF THE 42ND CONFERENCE OF THE JAPANESE SOCIETY OF APPLIED PHYSICS, Lecture No. 30a-ZH-8, 1995 (Spring 1995), Kobayashi et al. describe a process in which GaN:Mg is activated at a high temperature of about 800.degree. C. until the hole carrier concentration reaches in the range 4 to 8.times.10.sup.17 cm.sup.-3. GaN:Mg is GaN doped with magnesium (Mg). Then, gold/nickel metal electrodes are deposited and annealed. This produces a p-contact with a sheet contact resistivity reduced to 10.sup.-2 .OMEGA.cm.sup.2. However, this value is still insufficiently low.
Some conventional devices in which an electrical connection is made to a p-type Group III-nitride semiconductor a nickel electrode directly deposited on the p-type Group III-nitride semiconductor. Nickel is deposited on the exposed surface of the p-type semiconductor and is then annealed at a temperature above its eutectic point for a relatively short time. One or two minutes is typical. However, using nickel as the electrode material of a p-contact for a Group III-nitride semiconductor results in the p-contact having high resistance and a large voltage drop across the p-contact with the LED or LD incorporating such p-contact is driven. Consequently, although a Group III-nitride semiconductor-based light-emitting device that will radiate the desired luminous intensity can be made using nickel as the material of the electrode of the p-contact, such a device requires a high drive voltage, and an excessive amount of heat is generated at the p-contact. Such a light-emitting device has to be fitted with a heat sink to dissipate this heat. Moreover, the amount of heat generated by the resistance of the p-contact prevents the light-emitting device from being operated continuously, and impairs the service life and reliability of the device.
Other metals often used as the electrode material of the p-contact of Group III-nitride based light-emitting devices include gold (Au), platinum (Pt), nickel (Ni), and iridium (Ir), but acceptable results are not obtained using these metals.
If a low resistance p-contact for GaN could be fabricated inexpensively and with excellent reliability, it is likely that such a p-contact could also be used in many other semiconductor devices in which an electrical connection must be made to a p-type Group III-nitride semiconductor. Therefore, what is needed is low-resistance p-contact for p-type Group III-nitride semiconductors and a simple way of making such a p-contact. The p-contact should be usable in light-emitting devices, such as LEDs and LDs, based on Group III-nitride semiconductors, and should provide such devices with a low drive voltage, high efficiency, high reliability, and a long service life.